Control system for a continuously variable v-belt transmission

ABSTRACT

A continuously variable transmission, having a first pair of conical disks including an axially moveable disk and an axially non-movable disk whereby a running radius of the first pair of disks is changeable, a second pair of conical disks including an axially movable disk and an axially non-movable disk whereby a running radius of the second pair of disks is changeable, and an endless transmission element arranged to couple the first pair of disks with the second pair of disks. A first hydraulic actuator in operative connection with the movable disk of the first pair of disks for axially moving the disk, the first actuator having an inlet port, and a second hydraulic actuator is in operative connection with the movable disk of the second pair of disks for axially moving the disk, the second actuator having an inlet port. A first, four-quadrant controllable pump-motor has a first port connected to the inlet port of the first actuator, and a second port connected to the inlet port of the second actuator. The first pump-motor is operative to control a pressure ratio between the inlet ports of the actuators. A second, two-quadrant controllable pump-motor has an inlet port connected to a hydraulic medium source, and an outlet port connected to the inlet port of one of the actuators. The second pump-motor is operative to control tension in the endless transmission element. The output of the pump-motors is controlled and varied based on demand.

BACKGROUND OF THE INVENTION

The invention relates to a control system of the kind as described inthe preamble of claim 1 and as known as such from EP-A-0 423 536.

In this known transmission system the outlet port of the supply pumpwhich supplies the hydraulic medium under pressure from a medium supplyis connected via two one-way valves with the respective inlet ports ofeach of the actuators, which are thus connected with each other only viathe first hydraulic pump-motor and from which no hydraulic medium canflow back to the supply.

This known transmission has two principle drawbacks. The first drawbackis that this transmission can only work in a very narrow range oftransmission ratios at both sides of the transmission ratio: one. Thisresults from the fact that, when the transmission ratio deviates largelyfrom the value one, thus when the running radius of the transmissionmedium on the one of the pair of conical disks is small, and the same onthe other pair large, the adjustment of the conical disks—in which thedisks with the smaller running radius move towards each other and samewith the greater running radius move away from each other—the distanceover which the disks must be adjusted with respect to each other inaxial direction is unequal for the respective pairs of conical disks; ofcourse then also the respective amounts of displaced hydraulic mediumare unequal. In this know device, however, as a result of the one-wayvalves no hydraulic medium can flow back to the supply volume so thatthe control system will inevitably lock itself up when such adjustmentstake place.

A second drawback is the fact that the second hydraulic supply pump is asimple supply pump combined with a spring biased shunt valve, the pumpbeing driven constantly. As long as the transmission ratio does notchange all the energy supplied by the pump is converted in the biasvalve in useless heat. Thus the pump provokes very great energy losseswhich greatly reduce the total efficiency of the transmission.

SUMMARY OF THE INVENTION

The aim of the invention is to improve a control system of the kind asmentioned above in such a way that it can be operated over a very largerange of ratios with a minimum of losses. This aim is obtained with themeasures according to the characterizing part of claim 1.

As a result of the open connection between the outlet port of thetwo-quadrants controlled second pump-motor and the inlet port of one ofthe actuators medium can be supplied from, and returned to, the supplyvolume, ensuring that the in four-quadrants controlled, first pump-motorcan cover without any problems the whole range of ratios of thetransmission, while the fact that the second pump-motor can becontrolled in two quadrants results in that this second pump-motor mustonly set a minimum pressure level for one of the actuators, which has avalue only a little above the value with which under the presenttransmission ratio and torque conditions slipping of the transmissionmeans would occur, the volume flow which must be supplied is very small,not more than necessary to compensate the difference in volume flow fromthe one actuator to the other with added to it the internal and externalleak losses which might occur in the transmission or in the controlsystem. When the transmission ratio remains constant this volume flowremains restricted to only the amount which is necessary to compensateoccurring leak losses. As a result the energy consumed by the secondpump-motor will only be very small and the total efficiency of thetransmission will be increased in proportion thereto.

It is observed that the measures according to the invention result intoa transmission of which the control and supply system is made up withsimple, relatively cheap and compact, electric motors which, with thepresent prior art can be operated and controlled quite dependable. It isnot necessary any more to use the known hydraulic pump, driven by theengine which drives the transmission and which provokes quite heavylosses.

The preferred embodiment according to claim 5 has the advantage that thesecond pump-motor will always be connected to the actuator which carriesthe lowest hydraulic setting pressure so that this second pump-motor,irrespective of the direction in which the energy flow passes throughthe transmission and irrespective of the actual transmission ratio,needs only to supply the lowest pressure level, thus resulting in afurther reduction of energy taken up by this pump-motor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be elucidated on the hand of examples shown in thedrawings. In these drawings similar parts are indicated by the samereference numerals. In the drawings show:

FIG. 1 a first embodiment of a CVT provided with the control systemaccording to the invention;

FIG. 2 a schematic view of a CVT in which some of the various possiblepositions of the endless transmission means which runs between the pairsof conical disks are shown;

FIG. 3 a diagram which shows the connection between the running radii ofthe transmission means on the conical disks;

FIG. 4 shows a relevant part of a preferred embodiment of the controlsystem with an automatic setting valve according to the invention;

FIG. 5 a further embodiment of the valve shown in FIG. 4;

FIG. 6 another embodiment of the valve shown in FIG. 4;

FIG. 7 shows a relevant part of the still another embodiment of thecontrol system according to the invention;

FIG. 8 shows a relevant part of the further embodiment of the controlsystem according to the invention;

FIG. 9 shows a simple embodiment of the control system with CVTaccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a combination of a continuously variable transmission 1,and, beneath the dividing line 3, the control system 2 thereof. Thecontinuously variable transmission 1 is provided with a first pair ofdisks 4, comprising a disk 6 which is fixed to the shaft 5 and a movabledisk 8 which can move with respect to the shaft 5 by pressure in a firstchamber 7 in axial direction, indicated with the arrow x₁. The disk 8,with the piston surface 21 with surface A₁ coupled thereto is incombination with the chamber 7 the first setting means of the CVT 1. Theactuator is via the port 15 connected with the conduit 16 which goes tothe control system 2.

The CVT is furthermore provided with a second pulley with two disks 9,which are mounted on a shaft 10 and comprising the disk 11, fixed to theshaft 10, and a axially, in the direction of the arrow x2 displaceablesecond disk 13 being under the influence of the pressure in the secondchamber 12. The disk 11 with the piston surface 22 with surface A₂coupled thereto is in combination with the chamber 12 the secondactuator of CVT 1. This actuator is via the inlet port 17 connected tothe conduit 18 which goes to the control System 2.

The chambers 7 and 12 are filled with a suitable hydraulic medium, forinstance oil. Between the disks 6, 8 and 11, 13 there is an endlesstransmission means with a trapezoid shaped cross section 14. Thistransmission means can be a flexible strap, belt or chain or anon-flexible massive ring.

In FIG. 2 are shown the first extreme position “A”, an arbitrary inbetween position “B” and the other extreme position “C” which thetransmission means 14 can have between the pair of disks 4 and 9.Furthermore this figure shows the rotational speeds n₁ and n₂, thetorques T₁ and T₂ of the disk pairs 4 and 9. Furthermore this figureshows the running radii of the transmission means 14 on the pair offdisks. For the arbitrary intermediate position these are indicated withr₁ on the disks 4 and with r₂ on the disks 9. Generally r₁ can be bothsmaller and larger than r₂, while in a particular intermediate position,the middle position, the situation can occur that r₁ is equal to r₂. Thedisk pairs 4 and 9 are coupled to each other via the transmission means14. Apart from a small elastic stretch which will commonly occur duringthe operation the circumferential length L of the transmission meanswill remain constant. There is thus a nearly fixed relationship which isdetermined by the geometric structure of the CVT between the runningradii r₁ and r₂ and for this relation can be shown that it is given,with a good approximation, by the formula:${r_{2} = {r_{1} - {\frac{\pi}{2} \cdot a} + \sqrt{{\frac{1}{4}{\pi^{2} \cdot a^{2}}} - {2 \cdot a^{2}} + {a \cdot L} - {2{\pi \cdot a \cdot r_{1}}}}}},$

in which a is the center distance between the pair of disks, as shown inFIG. 2. This formula clearly shows that the relation between r₁ and r₂is not linear.

FIG. 3 is a graphic representation of this relation; the running radiusr₁ is taken along the horizontal axis and the running radius r₂ on anequal scale along the vertical axis. The respective running radii in theextreme positions are provided with the index min and max respectively.From this drawing can be concluded that during the change of thetransmission ratio, as defined by the ratio between r₁ and r₂ thesmaller of the two running radii changes more than the larger. On thepoint r₁=r₂ the tangent to the curve includes an angle of 45° and onlyaround this point the running radii will change in an equal amount. Inthis point the transmission ratio is one.

Influencing the tension in the transmission means 14 and thetransmission ratio of the CVT, as defined by the running radii of thetransmission means 14 on the pair of disks, is done by means of thecontrol system 2 by changing the pressure in the chambers 7 and 12 andby controlling the oil flow to and from these chambers. When changingthe transmission ratio in the one direction both disks 8 and 13 move tothe left, so that the volume of the chamber 7 increases and the volumeof the chamber 12 decreases. When changing the transmission ratio in theopposite sense both disks 8 and 13 move to the right, decreasing thevolume in chamber 7 and increasing the volume in chamber 12. The controlsystem 2 is via two conduits 16 and 18, which go to the chambers 7 and12, connected to the CVT 1.

The first embodiment of the control system 2 as shown in FIG. 1comprises the hydraulic pump-motor 19 with ports 20 and 23, connectedvia the conduits 16 and 18 respectively between the two chambers 7 and12. This pump-motor makes it possible to displace medium from chamber 7to chamber 12, and opposite. As a result the volume of the amount ofmedium present in the respective chambers will change and the disks 8and 13 which are connected to the chambers will move in axial direction,as a result of which the position of the transmission means with respectto the pair of disks 4 and 9 will change. When displacing medium fromchamber 7 to chamber 12 the transmission means will move in thedirection of the extreme position which is indicated in FIG. 2 by “C”,while when displacing the medium in the opposite direction thetransmission means will move in the direction of the other extremeposition “A”. In this way the transmission ratio, defined as the ratiobetween the running radii which the transmission means has on boththereof disks can be varied continuously.

It is observed that it is of great importance that the amount of mediummoved to and from chambers 7 and 12 respectively will generally beunequal to each other as a result of the herein above discussednon-linear relation between the running radii. The displacements in thedirection x₁ and x₂ of the movable disks 8 and 13 respectively areproportional to the changes of the running radii r₁ and r₂ respectivelyand bring about an unequal displaced volume. This is also the case whenthe piston surfaces A₁ and A₂ are mutually equal. Under somecircumstances the amount of medium which flows from one of the chambers7 or 12 can be more than the amount of medium which can be taken up bythe other chamber. Thus somewhere in the circuit which comprises thechambers 7 and 12, conduits 16 and 18 and pump-motor 19 there must be apossibility for the medium to flow back to the supply volume. In theknown device according to the prior art such a back flow is blocked bythe presence of one way valves so that under these circumstances achange of transmission ratio is only possible in a very limited rangearound the position in which r₁=r₂. In the control system according tothe invention this problem is solved by connecting the outlet port 24 ofa second hydraulic pump-motor 25 to one of the conduits 16 or 18 of saidcircuit, so that this pump-motor is connected directly (thus withoutone-way valve) to one of the ports 15 or 17 of the setting means of theCVT 1. In the present embodiment a connection to the port 17 is chosen.The other port 26 of the pump-motor 25 is connected via the conduit 27to the supply volume 28. Under circumstances in which medium must flowback to the supply volume the pump-motor acts as a hydromotor and sothat now the transmission ratio can be changed. When the amount ofmedium which leaves one of the chambers 7 and 12 is smaller than theamount of medium which can be taken up by the other chamber thepump-motor 25 acts as a pump and provides the missing amount. In thatcase medium flows via the conduit 27 from the supply volume 28 to theconduit 18. The pressure on the port 24 of the pump-motor 25 will alwaysbe higher than the pressure on the port 26, while the medium can flow intwo directions and in opposite direction (from port 26 to port 24).According to the proposal of this invention the pump-motor 25 is of atype which is controllable in two quadrants and has thus preferably acontrollable output. In such a pump-motor the possibility exists tocontrol the pressure at the outlet port 24 and to adjust this to theminimum necessary value which depends upon the load and the transmissionratio. When the circumstances are stationary, which means when there isa constant transmission ratio, the advantage is obtained that the outputcan be reduced to the small output necessary to compensate leak losseswhich occur in the CVT and the control system 2. Contrary thereto in theknown device according to the prior art under such circumstances thetotal output of the pump is converted in the overflow valve in uselessheat. As these losses do not occur anymore the efficiency of the CVT isincreased. Furthermore the pump-motor 25 can under those circumstancesin which it acts as hydromotor supply energy. The control system givesin principle the possibility to recover this energy, resulting into afurther increase of the total efficiency of the CVT.

In the pump-motor 19 the pressure at port 23 during operation can be, independence upon the present load and transmission ratio of the CVT,higher as well as lower as same on port 20, while, dependant upon thedirection in which the transmission ratio is changed, the flow can befrom port 20 to port 23 as well as opposite. The pump-motor 19 is forthis reason of a type which is controllable in four quadrants and haspreferably a controllable output so that by controlling the volume flowthe transmission ratio and the speed with which it changes can becontrolled accurately. When in use only the pressure difference betweenthe chambers 7 and 12 as such is present between the ports 20 and 23.When the pump-motor 19 acts as a pump relatively little energy will beconsumed, while in the other case, in which it acts as a hydromotor, inprinciple even the possibility exist to recover energy.

Preferably the pump-motors 19 and 25 have a fixed displacement and are,for instance, simple and cheap gear pumps. The pump-motor 19 is drivenvia the shaft 35 by means of a driving source 29 which is speedcontrolled and the pump-motor 25 is driven via shaft 36 by means oftorque-controlled driving source 30. These driving sources arepreferably of the electrical type and are controlled by means ofelectrical circuits 69 and 70 respectively by means of the commonlyknown electrical control units 31 and 32 respectively, of which theterminals 33 and 34 respectively are provided with a control signalwhich controls the rotational speed and the torque respectively. Thepressure difference over the pump-motor 25 is positive only while thevolume flow from this pump-motor can be both positive and negative. Thecontrol unit 32 is for this reason preferably a two-quadrants controlunit. Under circumstances in which there is a volume flow through thepump-motor 25 in the direction of the supply 28 this pump-motor will actas a hydromotor and the driving source 30 as a generator, so that thecontrol unit 32 can in principle supply electrical energy back. In thepump-motor 19 both the pressure difference and the direction of the flowcan be positive or negative. The control unit 31 is for this reasonpreferably a four-quadrants control and in principle electrical energycan be generated when the pump-motor 19 acts as a hydromotor and thedriving source 29 acts as a generator. In their most simple embodimentthe control units 31 and 32 respectively comprise a voltage and currentcontrol unit respectively.

Apart from the advantage of an increase of the efficiency of the CVT asa whole such a control system has the big advantage that it is simpleand can be implemented at low cost. Furthermore it is excellently suitedto be controlled by an electronic system which preferably comprises amicroprocessor, and which supplies to the terminals 33 and 34 thenecessary electrical control signals.

This electronic system is preferably connected to sensors which supplysignals representing the actual values of the transmission ratio and thetransmitted torque respectively. To determine the transmission ratiothese sensors preferably consist of rotational speed sensors coupledwith the pair of disks 4 and 9 or of a position sensor which is coupledwith one of the movable disks 8 or 13. To determine the transmittedtorque one will preferably use a torque sensor in one of the shafts 5 or10, or use a signal which is supplied by the engine which drives the CVTor by the load which is driven by the CVT. The rotational speed of thepump-motor 19 and the transmission ratio which is dependent thereon, orthe speed of change of the transmission ratio are controllable in asimple way in dependence upon the requirements of the energy source andthe load connected to the CVT. By means of the signals obtained from thesensors the pressure supplied by the pump-motor 25 can be adjusted tothe minimal necessary value by controlling the torque supplied by thedriving motor 30.

Dependent upon the use of the CVT it is an advantage that with a controlsystem as described above a further increase of the efficiency of theCVT as a whole is possible by making A₁ Unequal to A₂. For instance whenusing the system in a car, in which the pair of disks 4 is connected toan internal combustion engine and a pair of disks 9 is coupled to thedriving wheels the absolute pressure in the chanters 7 and 12, as wellas the pressure difference between these chambers will be highest in theposition “A” of the transmission means 14. Furthermore when acceleratingfull throttle the speed with which the transmission ratio changes willbe highest in the position “A” and, when the speed of the car increases,will smoothly decrease until position “C” is reached. As a result theamount of energy used by the control system will be highest in theposition “A”.

It can be deducted from FIG. 3 that when A₁=A₂, the amount of oil whichflows to chamber 7 is greater than the amount of oil which leaveschamber 12. When A₁ is greater than A₂ this difference decreases, sothat the volume to be supplied by pump-motor 19 decreases, whilefurthermore the pressure to be supplied by this pump-motor alsodecreases. By both these effects the energy which must be supplied bythis pump-motor decreases, so that the efficiency of the CVT as a wholeis still further increased.

FIG. 4 shows a relevant part of the preferred embodiment of the controlsystem according to the invention in which a still further decrease ofthe losses in the control system is obtained. The implementation andcontrol of the pump-motors 19 and 25 is equal to the ones alreadydiscussed on the hand of FIG. 1. In this embodiment however, one uses anautomatic selecting valve 37 parallel to the pump-motor 19, providedwith a supply port 38 supplied with medium under a basic pressure, by apump-motor 25. The switch-over valve 37 is furthermore provided withcontrol ports 39 and 40, connected to the conduits 16 and 18 and outletports 41 and 42, also connected to the conduits 16 and 18 and betweenwhich the pump-motor 19 is connected.

The control system 2 operates in these embodiments in such a way thatthe position of the valve body 43 which can shift in the bore 71 of thevalve housing 44 is determined by the pressure difference in thechambers 7 and 12, so that always the lowest of the pressure in thechambers 7 and 12 will be equal to the pressures on the supply port 38.This means that in the shown situations of FIG. 2 the pressure of thepump-motor 25 at the positions between “A” and more or less the middleposition, in which the pressure in chamber 7 is lower than the pressurein chamber 12 will operate in chamber 7 and in the area between themiddle position and position “C”, in which the pressure in chamber 7 ishigher than the pressure in chamber 12, in chamber 12. Thus the pressuresupplied by pump-motor 25 will automatically be present in the correctchamber 7 or 12. As the pressure of the pump-motor 19 is now under allcircumstances the lowest of these two pressures the energy taken up bythis pump-motor will be less and the efficiency of the CVT as a wholewill be increased. Furthermore the pressure of the pump-motor 25 isactive for nearly each position of the transmission means in the chamberof the pair of disks on which the transmission means has at this momentthe smallest running radius. On this pair of disks there is the greatestrisk of slipping of the transmission means and it is an advantage thatthe pressure in this chamber is directly controlled and can thus beadapted accurately to the momentary values of the transmission ratio andthe transmitted torque T₁ or T₂.

The dimensions of the valve body 43 are furthermore such that in themiddle position of the valve the supply port 38 via the space around thenarrower middle part 68 of the valve body is at least connected to oneof the, ring shaped, outlet port spaces 66 and 67 which are inconnection with one of the outlet ports 41 or 42. Furthermore the edgesof the valve are of such a shape that there is a smooth connection withthe outlet ports, for instance by providing the edges of the valve bodyor of the valve housing with beveled sides 45.

FIG. 5 shows a further embodiment of the selecting valve 37,particularly suitable to be used in those CVT systems in which therespective actuators of the pair off disks have unequal surfaces A₁ andA₂, in which then, for instance, the right hand head surface of thevalve body 45, in case A₁<A₂, is increased. By means of end switches 46and 47, on both the end surfaces of the valve body, signals can beobtained which indicate which of the chambers 7 and 12 at a particularmoment is connected with the pump-motor 25, and to adapt the controlsignal supplied to terminal 34 to the surface of the relative piston. Ofcourse each other kind of suitable position sensor can be used todetermine the position of the valve.

FIG. 6, shows an embodiment of the selecting valve 37 in which inparticular the leakage losses in the middle position of the valve body43 will be minimal. The dimensions of the valve body 43 are such thatthere is in the middle position no direct connection between the supplyport 38 and the outlet ports 41 or 42. In this position the pressuresupplied by the pump-motor 25 is brought to one of the outlet ports 41or 42 via the conduits 48 and 49 with therein the one-way valves 50 and51 which only allow a volume flow in the direction of the outlet ports.In this way a connection is made with the port which has at that momentthe lowest pressure.

FIG. 7 shows a relevant part of still another embodiment of the controlsystem 2 according to the invention. In this embodiment the first andsecond pump-motors are replaced by a third pump-motor 72 and a fourthpump-motor 77. The outlet ports 73 and 78 of the pump-motors 72 and 77are connected directly to the chambers 7 and 12 via the conduits 16 and18. In this case one pump-motor acts as pump-motor with which anindependent pressure level can be set in one of the chambers while theother pump-motor controls the transmission ratio by controlling thevolume flow. In both pump-motors the pressure at the outlet ports 73 and78 respectively is in use always higher than the pressure at the inletports 74 and 79 respectively so that both pump-motors are of the typewhich is controllable in two quadrants. Preferably this system comprisesa pressure difference sensor 52, for instance implemented by a valvebody 53 which can be shifted under the influence of the pressuredifference in the conduits 16 and 18 and which is combined with at leastone end switch 54, connected to a function indicator 55 by means ofwhich the functions of the pressure control and the control of thetransmission ratio can be delegated to either the pump-motor 72 or 77.If necessary the pressure control surfaces on both sides of the valvebody can be different. The operation of the function allocation elementis such that the function of controlling the pressure is allocated tothat pump-motor which is at that moment connected to the chamber withthe lowest pressure while the other pump-motor is connected to the otherchamber. In the case in which the pump-motors are electrical motors atorque control signal, supplied to terminal 56, will when the pressurein conduit 18 is higher than the pressure in conduit 16 be supplied tothe control unit 76 while at the same time a rotational speed controlsignal present at terminal 57 is supplied to the control unit 81. In theother case, in which the pressure in conduit 18 is lower than thepressure in conduit 16, the control signals supplied to the controlunits 31 and 32 are switched. The advantage of such a control signal isthat the pump-motor which controls the pressure is always connected tothe chamber in which there is the lowest pressure, thus resulting into asaving of energy. Furthermore this resulting embodiment, too, gives inprinciple the possibility to recover energy in the case in which mediumflow back via the pump-motors 72 and 77 to the supply volume 28 and thepump-motors act as hydromotor, so that the efficiency of the CVT as awhole is increased.

In an other embodiment based on this principle the pressure differencesensor 52 is omitted but the function allocating element 55 is via itsinputs 58 connected to sensors supplying a value which represents theactual value of the transmission ratio and the transmitted torque. Fromthese signals can be deducted on the hand of characteristics known assuch, in which chamber the pressure has the lowest value, after which afunction allocation to the correct pump-motor follows.

In still other embodiments of the control system according to theinvention the pump-motors 19 and 25 are implemented as pump-motor with avariable stroke-volume. Such pump-motors can be implemented as apump-motor, known from the prior art with at variable stroke-volume ofwhich under control of a supplied mechanical, electrical hydraulicalcontrol signal either the volume flow or the pressure, or one of thesevalues can be controlled. FIG. 8 shows schematically a relevant part ofa control system according to the invention with pump-motors 19 and 25,which are implemented in this way. Pump-motor 19 has a control unit 60,with an input 61 to which a volume flow control signal is suppliedwhich, by varying the stroke-volume controls the volume flow. Pump-motor25 is provided with a control unit 62 and with an input 63 which issupplied with a pressure control signal controlling, by varying thestroke-volume, the pressure. The pump-motors are preferably driven via acommon shaft 64 which can be coupled to the shafts 5 or 10 of the CVT.Otherwise this embodiment operates as described for the embodiment ofFIG. 4. An advantage is that under the circumstances in which the onepump-motor acts as a hydromotor and the other as a pump the energy whichis delivered by the one pump-motor can be used to drive via the commonshaft the other pump-motor so that the total amount of consumed energydecreases and the efficiency of the CVT as a whole is increased. Ofcourse the pump-motors 19 and 25 can be implemented in a similar way inthe embodiments according to the FIGS. 1-7 and 9. This embodiment too isvery well suited to be controlled by an electronic system, preferablywith a microprocessor, which supplies in this case the volume flow andpressure signals to the inputs 61and 63 respectively.

FIG. 9 shows a simple embodiment of the control system according to theinvention in which the pump-motor 25 is omitted. There is no hydraulicmedium in the chamber 12. It is replaced by the springs 65 with whichthe transmission means is tensioned. Pump-motor 19 is in this embodimentvia the port 23 connected directly to the supply volume 28 and is of thetype which can be controlled in two quadrants. By means of thepump-motor 19 the transmission ratio by the speed with which it changescan be controlled by supplying a rotational speed control signal toterminal 33 of the control system 31. The advantage is that the controlsystem 2 can be simple and cheap in case the difference between theutmost limit positions “A” to “C” is small and the load of the CVT orthe transmission ratio varies only a little during the operation of thetransmission. The losses in the control system are small so that theefficiency of the CVT as a whole is high.

What is claimed is:
 1. A continuously variable transmission, comprising:a first pair of conical disks including an axially moveable disk and anaxially non-movable disk whereby a running radius of the first pair ofdisks is changeable; a second pair of conical disks including an axiallymovable disk and an axially non-movable disk whereby a running radius ofthe second pair of disks is changeable; an endless transmission elementarranged to couple the first pair of disks with the second pair ofdisks; a first hydraulic actuator in operative connection with themovable disk of the first pair of disks for axially moving the disk, thefirst actuator having an inlet port; a second hydraulic actuator inoperative connection with the movable disk of the second pair of disksfor axially moving the disk, the second actuator having an inlet port; afirst, four-quadrant controllable independent pump-motor combinationhaving a first port connected to the inlet port of the first actuator,and a second port connected to the inlet port of the second actuator,the first pump-motor combination being operative to control a fluid flowbetween the inlet ports of the actuators; a second, two-quadrantcontrollable independent pump-motor combination having an inlet portconnected to a hydraulic medium source, and an outlet port connected tothe inlet port of one of the actuators, the second pump-motorcombination being operative to control tension in the endlesstransmission element; means for controlling and varying output of thesecond pump-motor combination in dependence on a required operatingtension in the endless transmission element; and means for controllingand varying a quantity and direction of fluid flow through the firstcontrollable independent pump-motor combination respectively to and fromthe first and second actuators in dependence on a required change in therespective running radii of the pairs of disks controlled thereby, boththe first and second pumps being driven discontinuously.
 2. Acontinuously variable transmission according to claim 1, wherein thefirst pump-motor combination comprises a combination of a hydraulicdisplacement pump coupled to a first electric motor, controlled by afour-quadrant control, and the second pump-motor combination comprises acombination of a hydraulic displacement pump coupled to a secondelectric motor, and controllable by a two-quadrant control.
 3. Acontinuously variable transmission according to claim 2, wherein thefirst electric motor is controlled in its rotational speed.
 4. Acontinuously variable transmission according to claim 2, wherein thesecond electric motor is controllable as to delivered torque.
 5. Acontinuously variable transmission according to claim 1, and furthercomprising a controllable selector valve arranged to connect the outletport of the second pump-motor combination to the inlet port of one ofthe actuators runs, the valve having outlet ports that are respectivelyconnected with the inlet port of one of the actuators, said valve beingcontrolled during operation so that an inlet port of the selector valveis connected with an outlet port of the two actuators which carries alowest pressure.
 6. A continuously variable transmission according toclaim 5, wherein the selector valve comprises a valve housing and avalve body slidingly accommodated in a bore in the valve housing, thevalve body having a narrowed central part, which, in a center positionlies between respective, ring-shaped outlet port spaces which connect tothe bore and communicate with an outlet port, and which also covers theinlet port which opens into said bore.
 7. A continuously variabletransmission according to claim 6, wherein the valve body has respectiveheads with surfaces that are mutually different, and further comprisinga position sensor coupled to the valve body, the selector valve beingarranged to cooperate with the position sensor, the position sensorbeing operative to produce an output signal that is used to controlpressure of the second hydraulic pump-motor combination in dependenceupon the position of the selector valve.
 8. A continuously variabletransmission according to claim 6, wherein a distance between end edgesof the narrowed central part of the valve body is greater than adistance of closest end edges of the outlet port spaces cooperatingtherewith.
 9. A continuously variable transmission according to claim 6,wherein a distance between end edges of the narrowed central part of thevalve body is no greater than a distance between closest end edges ofthe outlet port spaces cooperating therewith, each of the outlet portsbeing connected via a one-way-valve to the inlet port.
 10. Acontinuously variable transmission, comprising: a first pair of conicaldisks including an axially moveable disk and an axially non-movable diskwhereby a running radius of the first pair of disks is changeable; asecond pair of conical disks including an axially movable disk and anaxially non-movable disk whereby a running radius of the second pair ofdisks is changeable, the movable disk being acted upon by mechanicalforce; an endless transmission element arranged to couple the first pairof disks with the second pair of disks; a hydraulic actuator inoperative connection with the movable disk of the first pair of disksfor axially moving the disk, the actuator having an inlet port; atwo-quadrant controllable pump-motor combination having an inlet portconnected to a hydraulic medium source, and an outlet port connected tothe inlet port of one of the actuators, the second pump-motorcombination being operative to control ratio in the endless transmissionelement; and means for controlling and varying output of the pump-motorcombination based on demand so that the pump is driven discontinuously.11. A continuously variable speed transmission comprising a first pairof conical disks and a second pair of conical disks, mutually coupled bya tensionable endless transmission element, each pair rotatable around afirst and a second shaft respectively; one disk of the first pair beingmovable axially away from and towards the other, stationary, disk ofthis pair under the action of a first hydraulic actuator provided withan inlet port for a hydraulic pressure fluid, one disk of the secondpair being axially movable away from, and towards, the other,stationary, disk under the action of a second hydraulic actuator alsohaving an inlet port for a hydraulic pressure fluid such that, when themovable disk of the first pair is being moved towards the stationarydisk of said first pair the movable disk of the second pair is beingmoved away from the stationary disk of said second pair, and vice versa;the inlet port of the first actuator being connected by means of ahydraulic conduit to a first port of a pump of a four-quadrantscontrollable first hydraulic pump-electrical motor combination, theinlet port of the second actuator being connected by means of ahydraulic conduit to a second port of the pump of this hydraulicpump-electrical motor combination, and further provided with a secondhydraulic pump-electrical motor combination of which the pump has aninlet and an outlet, said combination being two-quadrants controllable,the pump inlet being connected to a source of hydraulic fluid while thepump outlet is connected to either the inlet of the first actuator orthe inlet of the second actuator, the arrangement being such that thetension in the endless transmission element is brought about by ahydraulic pressure generated by mean s of the two-quadrants controllablesecond hydraulic pump-electrical motor combination while the position ofthe movable disk of the first pair on the one hand and the position ofthe movable disk on the other hand is determined by the pressure ratioon the respective inlets of the first and the second actuatorrespectively, such as determined by the, four-quadrants controllable,first pump-electrical motor combination, and further comprising ahydraulic switching valve having a valve body with a cylindrical bore,closed at each end, which accommodates slidingly an elongate cylindricalplunger with a narrowed central part bounded by two shoulders, saidplunger defining a first and a second end chamber respectively betweeneach of its outer ends and a respective, opposite, end wall of the valvebody, the first end chamber having a port which connects to the inletport of the first actuator and the second end chamber having a partwhich connects to the inlet port of the second actuator, a port, openinginto the bore at the midpoint between the ends thereof being connectedto the outlet of the pump of the second hydraulic pump-electrical motorcombination and further comprising two circumferential grooves formed inthe surface of the bore symmetrically with respect to said midpoint andat a mutual distance which is greater than the distance of the shoulderswhich bound the narrow central plunger part, each of said grooves beingconnected to the port of its most adjacent end chamber.